High-speed optical fiber communication systems operate by encoding information (data) onto lightwaves that typically propagate along optical fiber paths. Most systems, especially those used for medium to long distance transmission, employ single mode fiber. As implied by the name, single mode fibers optimally propagate one mode of a lightwave. The single mode of light typically comprises many communications channels. The many communications channels are combined, or multiplexed into the one transmitted mode, as by wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM).
While there is only one mode transmitted, that single mode actually comprises two perpendicular (orthogonal) polarization components. These two components propagate at different speeds along a fiber transmission path, producing undesirable distortion of the optical signals referred to as polarization mode dispersion (PMD). PMD can be corrected in optical transmission systems using measurements of PMD to control active corrective optics.
Polarimeters measure the polarization of light, and PMD compensators correct dispersion. Polarimeters generate signals representing a measured state of polarization that can then be used for polarization correction. PMD compensators accomplish PMD correction based on the measured signals. These devices can be compactly fabricated as integrated structures on a substrate.
An important component in polarization measurement and polarization compensation is a component to modify the polarization of the light. This component is typically provided as a polyimide half waveplate inserted into a grove formed in a silica-based single mode waveguide. (See for example, “Polarization Mode Converter with Polyimide Half Waveplate in Silica-Based Planar Lightwave Circuits,” Y. Inoue, et al., IEEE Photonics Technology Letters, vol. 6, no. 5, pp. 626-628, 1994.) The waveplate is provided by dicing or etching a trench across a waveguide and then epoxying the half-wave plate in the gap. The difficulty with this approach is that the trench often cuts across other waveguides, adding unnecessary loss and backreflection. Also, typical polyimide half-wave plates have 40 micron widths. While, such widths are acceptable for low index waveguides, with high index waveguides the several decibel (dB) diffraction loss is not tolerable.
Accordingly there is a need for an improved component to modify the polarization of light, especially for use in polarization measurements and PMD compensators.